I don't brood my chickens and ducks together. I do however have the two turkeys with the chicks at the moment. Right now with the small ones I have two brooder totes in the house (hoping to get them out to the barn SOON), one for 6 ducklings and one for 2 turkeys, 12 chicks. For now everyone is about a week to two weeks old and it is working fine. Last year I think the ducklings were 5 weeks old when I put them with the chickens (adult), we had a bit of pecking the first week but it settled down pretty quickly and now the ducks and chickens are fine together.
I must say please figure out a plan for water if you are planning them together. What I do is either have water just outside or in a special part of the barn so that the bedding doesn't get wet. I also have the feed in another part of the barn so it doesn't get wet. The ducks will splash all water out. I believe in my smaller pen they are using 10 gal of water a day- 6 ducks, 17 chickens. Chickens when by themselves were using about a gal/day at most 2 gal/day. Which for me spells HAULING lots of water. They are also making that nice pen muddy, which it wasn't with just chickens. Sigh, it is time I rethought water again!